Analyzing Neutral Conductor and
Transformer Overloads
Power System Engineering
Solutions
Harmonic Concerns at a Battery Charging Plant
The Problem
The plant engineer at an automobile battery charging facility needed to
replace the aging 600 V bus duct in the plant. He was struggling with
conflicting claims from bus duct vendors about the use of 200% neutral
conductor rating in the proposed bus duct. He knew that battery chargers
produce significant amounts of harmonic currents, but was it necessary
to specify 200% neutrals… and pay the price premium?
As if the neutral issue wasn’t enough, the subject of harmonic currents
led to a discussion of transformer loading: what are the effects of these
harmonic currents on the 2500 kVA transformer serving the battery
charging circuits?
With this dual concern about harmonic distortion, the plant engineer
turned to Square D® Power System Engineering for help. Fortunately, his
plant is equipped with Square D® PowerLogic® circuit monitors at the
2500 kVA transformer and PowerLogic software. Figure 1 shows a fourcycle captured waveform from this circuit monitor. As the plant
engineer suspected, both voltage and current were badly distorted.
Key Concepts and Terms
Neutral Currents
A widely-discussed concern with harmonic-producing loads is that they
cause overloaded neutrals. These overloads occur because singlephase, 120 V power electronic devices produce harmonic current which
does not cancel on a shared neutral conductor. Because they operated
under the cancellation premise, outdated wiring systems allowed shared
neutral conductors on 3-phase circuits to be smaller than phase wires.
The problem was worsened by the fact that the neutral conductor, not
protected by circuit breakers or fuses, could fail even when phase
conductors were not overloaded.
Figure 1: 4-cycle captured
waveform illustrating voltage
and current distortion.
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Power System Engineering Solutions 1
Neutral (continued)
Figure 2 shows the neutral current resulting from a circuit loaded with
personal computers. The maximum amount of neutral current for a
personal computer load is 173% of the phase, although few systems
reach this theoretical limit. For badly distorted phase currents (above
32%), the highest neutral currents occur with well-balanced phase
currents. This is exactly the reverse of the harmonic-free state. Without
harmonics, the current load on a shared neutral conductor is related to
the amount of unbalance among the phase conductor currents.
So, why aren’t commercial buildings burning to the ground all over the
country? To answer this question, the National Electrical Code (NEC)
formed an ad hoc committee to investigate the need for oversized
neutrals. The committee found that neutrals sized to the same ampacity
as phase conductors are overloaded in only about 5% of the cases.
This is due, in part, to the fact that few 208/120 V systems are operated
at full rated capacity. So, even if the neutral current exceeds the phase
current, the conductor ampacity is not exceeded. The second reason
for the small number of actual failures is explained by another NEC
requirement for phase wires. This rule says that phase conductors (and
overcurrent protective devices) cannot be sized smaller than 125% of
their continuous load. Since the neutral is sized the same as the phase
conductor in most cases, the real current-carrying capacity of the neutral
is 125% of the continuous current in the phase wire.
Transformer Loading
Figure 2: Normal current on a circuit loaded
with personal computers.
Standard transformers are designed to supply their full rated capacity
under limited harmonic conditions. The standard to which power
transformers are designed says that they will transmit rated load at rated
temperature rise when:
• Energized from voltages with less than 1% total harmonic
distortion (THD)
• Serving loads with less than 5% THD current
In today’s electronic world, these two conditions are often exceeded.
The problem is assessing the effects of high harmonic distortion on
standard transformers.
1800
1600
Average Phase Current
Eddy-current loss in winding conductors is proportional to the square
of the load current that produces the field, and to the square of the ac
frequency of the current. Since harmonic currents contain high frequency
components, the eddy-current losses associated with harmonics can
cause a transformer’s design temperature rise to be exceeded before its
nameplate kVA rating is reached.
2 Power System Engineering Solutions
Current, Amperes
1400
When transformer windings carry ac current, each conductor is
surrounded by an ac magnetic field whose strength is directly
proportional to the magnitude of the current. Each metallic conductor
linked by the flux experiences an internal voltage that causes eddy
currents to circulate in the conductor. The eddy currents are dissipated
in the form of heat, producing an additional temperature rise in the
conductor. This type of extra loss in the windings is referred to as
eddy-current loss.
1200
1000
800
600
400
Neutral Current
200
0
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Wednesday
Figure 3: Average phase current and neutral
current of a 2500 kVA transformer
The Solution
The plant engineer was correct to be concerned about the effects of
harmonics on the proposed bus duct. However, the maximum neutral
current at his plant does not exceed the phase current. In fact, there are
no commercially available loads which contain enough triplen (multiples
of three times the fundamental frequency) harmonic currents to require
more than 100% neutrals at the 480 V level or above. The neutral
overloading issue is limited to 208/120 V switch-mode power supply
loads such as personal computers.
Table 1
Neutral Current Makeup (Amperes, rms)
Total
Phase
Current
H1
H3
H5
A
1093
844
281
253
B
997
798
233
217
C
1036
862
278
250
Neutral
804
50
793
26
Square D Power System Engineering verified its analytical predictions
with actual data from the battery charging plant. Square D engineers
reviewed the circuit monitor data from the PowerLogic software. Figure
3, on page 2, shows the average phase current and resulting neutral
current as measured at the 2500 kVA transformer. Note that the neutral
current does not exceed the average phase current. Table 1 illustrates
the neutral current makeup. These readings were obtained from the
PowerLogic software. Note that the fundamental neutral current (H1) is
small compared to the phase fundamental currents. The fundamental
component of the phase current almost cancels on the shared neutral.
The fundamental current on the neutral represents the unbalance of
the fundamental component on the phase wires. The fifth harmonic
component (H5) also represents the unbalance of H5.
Adds
Cancels
The third harmonic component (H3), however, adds on a shared neutral.
In fact, the predominant component on the neutral is third harmonic, or
180 Hertz. As figure 5 shows the neutral current frequency is three times
that of the phase current.
If you are concerned about neutral overloading on a 208/120 V circuit,
what should you do? One common solution is to increase the neutral
current-carrying capacity. This can be accomplished by running a
separate neutral wire for each phase wire, thus eliminating shared
neutrals. Another way of increasing the neutral ampacity is to replace a
shared neutral with one rated for 200% of the phase current. The neutral
buses on panelboards and transformers may need to be increased as
well, but keep in mind that only 4-wire, 208/120 V circuits are subject to
neutral currents that exceed phase currents. While the neutral concern
is easy to verify with power monitoring, the transformer loading issue is
more complicated. IEEE/ANSI Standard C57.110-2008 offers a method
to calculate the effect on transformer life of harmonic currents above 5%.
The method has been used to de-rate a standard transformer based on
the amount of harmonic current it must transmit; but de-rating a standard
transformer is not recommended as a permanent solution.
Figure 4: 4-cycle waveform capture from
PowerLogic software.
Phase A Current
2000
1000
0
-1000
-2000
0
20
Time (mS)
40
60
40
60
Neutral Current
2000
1000
Using an estimated eddy-current loss based on the battery plant
transformer type and design, Square D Engineers calculated the true
capacity of the transformer. Since the harmonic loading changes continuously, the estimated capacity of the transformer changes proportionately. Figure 6 (on page 4), illustrates the results of the calculation.
The best method for dealing with harmonic currents in power
transformers is to install power transformers designed to transmit more
than the 5% current THD limit. K-factor transformers are manufactured
for this purpose.
0
-1000
-2000
0
20
Time (mS)
Figure 5: Badly distorted phase current (upper)
at battery plant contains triplen components
that show up on the shared neutral (lower).
Power System Engineering Solutions 3
The Solution (continued)
The k-factor rating is related to the harmonic content of the expected
load. In fact, it is calculated from the harmonic current measured
in the load.
K-rated transformers typically have additional coil capacity to reduce
eddy-current losses. They are typically wound 480 V delta primary and
208/120 V secondary, with an electrostatic shield, and reduced core flux
to compensate for harmonic voltage distortion. Since these transformers
usually serve computer loads, they also contain double-size neutral
terminals to accommodate extra neutral wires. The higher the k-rating, the
more harmonic current the transformer can transmit; k-4 or k-13 ratings
are adequate for most circuits.
Conclusions
2500
Derated Capacity
2000
Load or capacity, kVA
After the Square D study, the plant engineer felt comfortable purchasing
new bus duct with a neutral ampacity of 100%. At 600 V, the battery
charging circuits do not inject enough triplen currents to cause excessive
neutral loads. Shared neutrals serving 208/120 V circuits, however, can
be subjected to currents which exceed the phase current. While it is good
practice to maintain well-balanced 3-phase circuits, the neutral problem
cannot be solved by improving the current balance among the phase
conductors. Better balance only increases the neutral current for badly
distorted phase currents. (Figure 5 on page 3), badly distorted phase
current (upper) at battery plant contains triplen components that show up
on the shared neutral (Figure 6).
1500
1000
500
Actual Load
Dealing with the transformer loading issue was more difficult. As a
stopgap measure, the plant engineer closed a tie circuit breaker on this
double-ended substation (to share the harmonic loading among two
transformers). This temporarily reduced the overloading concern, but can
lead to worse problems and is generally not recommended. The plant has
not decided on a long-term solution. The only certainty is that
catastrophic failure of transformers or bus duct would cost millions of
dollars in damaged product and lost sales. Facing that economic risk, the
plant will most certainly decide on a permanent solution.
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Figure 6: Square D calculation, illustrating the
true capacity of the transformer.
Additional Reading
Schneider Electric - North American Operating Division
295 Tech Park Drive
LaVergne, TN 37086
Tel: 866-466-7627 Toll Free
PowerLogic.com
Document# 3000HO0824
4 Power System Engineering Solutions
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These two documents give more information about harmonics and
neutral currents:
• Harmonics and Electrical Equipment Neutrals,
order number 0104PD9501
• Neutral Currents in Three Phase Wye Systems,
order number 0104ED9501
To review any of these documents, visit the Technical Library at
www.Squared.com